Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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A contraction of "synthetic tannins," which are chemicals that combine with, or affect, the protein constituents of hides and skins and produce a product that is flexible, porous, and has the desirable qualities of leather. The most widely known syntans are made by treating aromatic substances, e.g., cresols, phenols, naphthalenes, etc., with formaldehyde and sulfuric acid. There are many variations in the ingredients of syntans, relative quantities used, and methods of manufacture. Syntans produce white or buff-colored leather, depending on the ingredients, which darken upon exposure to light, and generally behave much like vegetable-tanned leathers. Although syntans do exist which can be used alone to produce leather (so-called exchange or replacement syntans), many syntans lack the filling power of vegetable tannins and produce an undesirably thin, "papery" leather. They are also more expensive than the natural tannins. Syntans do have desirable properties, however, and are widely used in both chrome and vegetable tannages. When used in conjunction with other tanning agents, where they are known as "auxiliary syntans," they perform the following functions: 1) the presence of 5% syntan helps dissolve solid vegetable tannin extracts and reduces any tendency to form REDS (condensed tannins) or BLOOM (pyrogallol tannins); 2) a pretannage with 5 to 10% syntan improves the shade, i.e., makes it paler, and the levelness of color of a subsequent vegetable tannage; 3) a pretannage with a syntan or admixture with a vegetable tannage improves penetration of tannin into the skin; 4) when syntan is used with a vegetable tannin the leather develops a more uniform but paler color upon being dyed, hut the syntan generally prevents the development of deep, full shades: 5) some types of syntan may be adjusted with an alkali to a pH of 6.0 to become "neutral syntans," often called synthetic mordants (but should be called "synthetic leveling agents") which have value in dyeing leather: and 6) some syntans retard mold growth and/ or remove iron stains. (248 , 291 , 306 , 363 )

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